The research proposed combines features of both basic and applied research in the field of mental retardation (MR). The explicit goal of this research is to increase understanding of mental retardation processes as well as provide novel approaches to patient classification. Two programs of research are described: 1. Studies of the sensory evoked response (SER). This is perhaps the most basic research effort of the two programs. Patients from a variety of diagnostic categories will be studied. SERs to sensory stimuli will be recorded. Stimulus parameters will be varied in intensity, duration, complexity, and meaningfulness. SER data will be used to define the dimensions in which evoked neural activity in retardates differs from non-retardates. SER patterns will also be used to study differences between MR diagnostic categories. 2. Development of EEG and SER patterns. A large number of patients will form a cross-sectional sample to assess developmental trends in EEG and SER patterns. Data will be processed by computer to determine auto- and cross-spectra as well as other meaningful descriptors of EEG activity. Also, a smaller longitudinal sample will be followed to determine developmental trends within the same subjects.